1. Field of the Invention
This invention relates to an improved hydrocarbon conversion process. More specifically, it relates to an improved process for the isomerization of alkylaromatic hydrocarbons.
2. General Background
Isomerization of xylenes is industrially performed by the steps, in suitable combinations, of isomerizing an aromatic hydrocarbon feedstock containing mainly xylene isomers, separating a specified xylene isomer, normally paraxylene, from the resulting isomerization reaction mixture, and recycling the mixture left after separation. It is industrially significant in this case, for increased efficiency of the isomerization reaction and a reduced cost of production, to adjust the composition of the xylene isomers in the isomerization reaction product as closely as possible to the thermodynamic equilibrium composition, and to inhibit side-reactions such as the decomposition of xylenes by hydrogenation of the benzene ring, dealkylation of a methyl group, or transalkylation.
Many methods for isomerizing xylenes have been suggested in the past and many of them involve the use of a crystalline aluminosilicate zeolite-containing catalyst. Crystalline aluminosilicates, generally referred to as zeolites, may be represented by the empirical formula: EQU M.sub.2/n O.Al.sub.2 O.sub.3.xSiO.sub.2.yH.sub.2 O
in which n is the valence of M and x is generally equal to or greater than 2. Zeolites have skeletal structures which are made up of three-dimensional networks of SiO.sub.4 and AlO.sub.4 tetrahedra, corner linked to each other by shared oxygen atoms. Zeolites particularly suited for use as isomerization catalysts include mordenite and the ZSM variety. In addition to the zeolite component, certain metal promoters and inorganic oxide matrices have been included in isomerization catalyst formulations. Examples of inorganic oxides include silica, alumina, and mixtures thereof. Metal promoters, such as Group VIII or Group III metals of the Periodic Table, have been used to provide a dehydrogenation functionality. The acidic function can be supplied by the inorganic oxide matrix, the zeolite, or both.
A commercially viable isomerization process is one that concurrently meets the following objectives. First, the process must exhibit high xylene isomerization activity and, second, it must produce the desired product without a significant loss of xylenes. This loss is a result of undesired side-reactions, involving hydrogenation of the aromatic ring, hydrogenolysis, demethylation, and particulary disproportionation and transalkylation.
Another factor of importance in a xylene isomerization process is the effect that ethylbenzene has on the entire isomerization and xylene recovery loop. When ethylbenzene, which is normally present in 8-carbon-atom aromatic fractions, is present in appreciable quantities in the feed to the isomerization process, it will accumulate in the loop unless it is excluded from the feed or converted by some reaction in the loop to products which are separable from xylenes by means tolerable in the loop. Ethylbenzene can be separated from the xylenes of boiling point near that of ethylbenzene by extremely expensive "superfractionation". A more desirable method of eliminating the ethylbenzene is through a conversion reaction taking place simultaneously with the isomerization reaction of the xylene. It is preferable that this ethylbenzene conversion reaction be a deethylation reaction producing benzene and ethane rather than a disproportionation reaction to benzene and diethylbenzene. The deethylation reaction preserves more xylenes and produces a high-quality benzene-containing reaction product.
It has now been found that, if a catalyst is formulated with the components and in the manner set forth hereinafter, an improved process for the isomerization of non-equilibrium mixed xylenes containing ethylbenzene is obtained.